Alkyl esters, including methyl ester, ethyl ester, propyl ester, butyl ester, and/or pentyl ester also known as biodiesel, are a renewable and clean burning alternative to conventional petroleum-derived diesel fuel. Biodiesel is made from a raw or used vegetable oil or animal fat, which is converted to the corresponding alkyl esters. Because biodiesel is made from natural oil or fat sources (i.e., natural source oils), the alkyl esters typically comprise C14 to C18 fatty chains if derived from vegetable oil, and C16 to C22 fatty chains if derived from animal fat. Biodiesel can be combusted in diesel (combustion-ignition) engines either in pure form or as blended with petroleum-derived diesel fuel. Biodiesel provides the benefits of a renewable resource as well as providing lower sulfur emissions than petroleum diesel. Biodiesel is effectively a zero-sulfur emission fuel.
The cost of feedstock for the production of biodiesel varies depending on the composition of the feedstock and may be a significant contribution to the final cost of the fuel product. Hence, it would be desirable to use lower-cost oil feedstocks that contain higher levels of free fatty acid and reduce the overall costs associated with the production of biodiesel. However, alkyl esters are commonly made by processes that require highly refined vegetable oils. For example, one process for the production of biodiesel employs the base-catalyzed transesterification of triglycerides with an alcohol such as methanol. In this process, a homogeneous catalyst (i.e., one that dissolves into the feedstock mixture) is used. Base-catalyzed transesterification when compared to acid-catalyzed transesterification typically has a higher reaction rate for converting a glyceride composition to alkyl esters. Unfortunately, these base-catalyzed transesterification processes cannot typically tolerate high levels of free fatty acid because of the soap by-product which is produced. The formation of a soap by-product may complicate product recovery efforts and purification and irreversibly consume a portion of the catalyst. Therefore, base-catalyzed transesterification processes typically require a highly refined natural source oil feedstock that is considerably more expensive than lower grade unrefined oil feedstocks.
Consequently, there remains a need in the art for methods of making alkyl esters using lower-cost feedstocks comprising a high free fatty acid content.